The shaping of these refractory objects involves setting the refractory grains into a stable shape termed a "green" shape prior to final sintering either as a separate operation or in situ on first firing of the furnace or similar operation. There are three principal ways in which a "green" shape can be formed. The first and the one with which the invention is primarily concerned may be termed a chemical formation and here the refractory grains are mixed with a liquid binding system to form a slurry. The slurry is formed to the desired shape and the system reacts chemically to set. Typically the slurry is aqueous and the reaction is a hydrolysis followed by setting of the hydrolysate by the formation of a polymer. The second possibility is dry or isostatic pressing; here the grains are only slightly moistened and pressed into the desired shape. Thirdly cement-based systems may be set hydraulically into a green shape.
The first "chemical" systems have been typified in the prior art by the use of binding liquids based on the alkoxides of silica and alumina. Systems based upon silica are however not suitable for binding basic refractories such as magnesite as the acidic silica tends to react with the basic refractory. The steel industry is coming to dictate non-acidic refractories which are less reactive with molten metals and slags.
There is therefore an increasing demand for basic refractories and our EPA No. 0,351,084 proposes mixing basic refractory grains with a binding liquid containing a phosphate preferably in the form of an ammonium polyphosphate. The phosphate reacts with the basic grains to set the slurry into a stable green shape.
However such systems have not been entirely satisfactory and pre-firing to a high temperature has been a necessity which has been a substantial economic and technical disadvantage.